| Season | Key biological events |
|---|---|
| March–April | Smelt migrate upstream to spawn around Putney – Battersea; Dace mass-spawn in Wandsworth; First elvers (~65 mm) arrive. |
| May | Dace larvae (~9 mm) drift downstream; Flounder post-larvae (>8 mm) appear in multiple waves; Smelt fry (~18 mm) present; Bream recruitment begins. |
| June | Sand smelt spawn in the Greenwich area; Common goby fry (>7–9 mm) appear; First bass fry waves (>12 mm) enter the estuary. |
| July–August | Continued waves of bass fry; High abundance of juvenile goby, flounder, and bass throughout mid–upper estuary. |
| September | Thin-lipped grey mullet fry (>15–20 mm) arrive; Juvenile bass, smelt, goby and flounder penetrate upriver to Chiswick–Richmond. |
| October–Winter | Progressive downstream migration of estuarine and marine species; Freshwater species move downstream as temperatures drop; Adult marine species exit to the North Sea. |
| Area | Species |
|---|---|
| Richmond | Bream, Perch, Roach, Bass fry, Flounder, Goby |
| Brentford | Bream, Perch |
| Putney – Battersea | Dace, Smelt |
| Chelsea creek | Roach |
| Greenwich | Sand smelt |
| Upper Thames (general) | Goby, Flounder, Smelt fry, Bass fry |
| Dock basins | Smelt |
| Area | Species | Why this area matters |
|---|---|---|
| Richmond – Brentford | Dace, roach, perch, bream; juvenile goby; bass fry; flounder | Primary nursery zone for freshwater and euryhaline juveniles; shallow margins and low salinity provide refuge and feeding habitat. |
| Putney – Battersea | Dace, Smelt | Confirmed major spawning corridor: dace (April) and smelt (March–April); crucial upstream migration route. |
| Greenwich | Sand smelt, smelt fry | Likely spawning area for sand smelt. |
| West Thurrock & below | Sole, plaice, dab, juvenile bass | Major flatfish nursery and important marine–estuarine interface; high abundance of juvenile marine fishes. |
| Gravesend & below | Adult smelt; marine juveniles | Primary entry corridor for marine species; starting point of smelt upstream spawning migration. |
| Dock basins | Smelt; mixed freshwater & estuarine species | Provide refuge habitats with stable temperatures and reduced flow; important overwintering and nursery areas. |
| Soft marginal vegetation | Roundfish fry: dace, smelt, bass | Essential microhabitat for fry using selective tidal stream transport; supports feeding and shelter; increasingly rare. |
| Continuous tidal foreshore | Dace fry, smelt larvae, bass fry | Required for uninterrupted fry migrations; heavily reduced (<1% of natural bank remains) due to encroachment and hard engineering. |
The Thames Estuary: a decade-by-decade story
1920s–1950s:
In the early 20th century, the Thames Estuary was heavily polluted by sewage and industrial waste. Oxygen levels were often near zero, and by the late 1950s long stretches of river between Kew and Gravesend contained almost no fish at all. A few species survived only in isolated places such as dock basins or marsh pools, where conditions were slightly better. These decades mark the ecological low point of the Thames.
1960s:
Major improvements to sewage treatment led to the first real rise in oxygen levels. By the late 1960s, scientists using power-station intake screens recorded 41 species, including smelt, shad, lamprey and eels — species that had vanished for decades. This was the first solid evidence that the Thames was beginning to recover.
1970s:
The 1970s marked a turning point for the Thames, as scientists began collecting regular, quantitative data on fish and shrimp for the first time. These surveys revealed strong seasonal patterns: marine species moved into the estuary during winter, while juveniles of many species appeared in large numbers each summer. However, the recovering system was quickly tested by major environmental events.
In 1970, a sewage workers’ strike caused dissolved oxygen levels to collapse, leading to widespread fish mortality and temporarily erasing several years of recovery. Just a few years later, the extreme drought of 1976 pushed salty water far upriver, creating conditions that allowed marine species such as gobies and flounder to reach central London — something never previously recorded.
By the end of the decade, these contrasting events showed that although chronic pollution was easing, the estuary remained highly vulnerable. Salinity, freshwater flow and hydrological extremes had become the dominant forces shaping where fish could survive and how far they could move.
1980s:
The 1980s marked a major step forward for the Thames, driven by large upgrades to sewage treatment at Crossness and Beckton. With cleaner water and more stable oxygen levels, fish communities continued to expand. Species such as eels, flounder, whiting, herring and smelt all increased in number as the river became a more reliable place to live and feed.
Scientists were also able to study the ecology of key species in detail for the first time. A landmark study of smelt revealed how they grew, fed and reproduced in the recovering estuary. At the same time, the appearance of rare visitors — including seahorses and occasional oceanic migrants—showed that the river was becoming hospitable to a wider array of species.
1990s:
Despite dramatic improvements in water quality, the 1990s revealed that the Thames Estuary was still fragile and easily disrupted. Heavy rain and sewage overflows caused sudden drops in oxygen, leading to fish kills. In response, the Thames Bubbler was introduced, a specialist vessel that pumps oxygen into the river during emergencies to prevent mass mortality.
Eel numbers continued to rise, but scientists raised concerns about poor juvenile recruitment and barriers that blocked their upstream migration. At the same time, surveys showed that although the water was cleaner, the riverbed sediments still held extremely high levels of heavy metals such as lead, cadmium and mercury — pollution left over from decades of industrial use.
Fish monitoring recorded around 60 species during this decade, with strong seasonal migrations and important nursery areas emerging in places like Richmond, Greenwich and West Thurrock. By the end of the 1990s, the Thames was no longer “biologically dead,” but a recovering ecosystem that required ongoing management — especially to safeguard nursery habitats and maintain healthy freshwater flows.
Araujo, R. (1998). Seasonal and between-year variations of fish populations in the middle Thames estuary (1980–1989). Journal of Fish Biology, 53(6), 1130–1147.
Thomas, M. (1998). Temporal changes in the movements and abundance of Thames estuary fish populations. In: Attrill, M.J. (eds) A Rehabilitated Estuarine Ecosystem. Springer, Boston, MA.
Araujo, R., Bailey, R., & Williams, P. (1999). Spatial and temporal variability of fish populations in the upper Thames estuary. Marine Biology, 135, 123–137.
Colclough, S. R., Gray, G., Bark, A., & Knights, B. (2000). Tidal Thames Fish Survey 1994–1996: Summary Report. Environment Agency / National Rivers Authority Thames Region.
Colclough, S. R., Fonseca, L., Astley, T., Thomas, B., & Watts, W. (2002). Fish and fisheries of the tidal Thames: Management of the modern resource, research aims and future pressures. Journal of Fish Biology, 61(sA), 64–73.
Zoological Society of London. (2019). Larval and juvenile fish in the tidal Thames: Baseline survey report. Prepared for the Thames Tideway Tunnel project. Tideway London.